Group IB-IIIA-VIA compounds, especially copper indium diselenide (CIS) and copper indium gallium diselenide (CIGS) compounds, are promising and much studied semiconductor materials for a number of thin-film semiconductor applications, not the least of which include absorber layers for solar cells, because the elements themselves are common and fairly inexpensive, and significant solar to electrical energy conversion efficiencies with CIS and CIGS absorber layers have been demonstrated. However, techniques for inexpensive production of consistently high quality Group IB-IIIA-VIA thin film semiconductor compounds needed for large scale commercial acceptance and applications have been elusive.
In general, the stoichiometry of Group IB-IIIA-VIA compounds that have exhibited the highest solar to electrical energy conversion efficiencies have been slightly poor, or at least not rich, in the Group IB component in relation to the Group IIIA component(s), which is sometimes called the I/III ratio. For example, in Group IB-IIIA-VIA compounds in which the Group IB component is copper, I/III ratios in a range of about 0.8 to 1.0 have demonstrated the best solar to electrical conversion efficiencies. When the I/III ratio exceeds 1.0, the Group IB-IIIA-VIA compounds become unstable and Group IB-VIA phases tend to form, which deteriorates the electrical characteristics of the material and adversely affects the solar energy conversion efficiency. Also, in general the larger the crystal grains of the Group IB-IIIA-VIA compounds, the better the electrical characteristics, which are directly related to the solar to electrical energy conversion efficiencies.
Unfortunately, such compositional control and uniformity with large grain polycrystalline film formation of Group IB-IIIA-VIA compounds over large substrate surface areas has not been easy to achieve. Most current CIS and CIGS production methods involve some kind of vacuum deposition technology, for example, physical vapor deposition, which typically has high capital costs as well as high production, costs. Therefore, there has been a long-time need for less expensive alternatives for producing consistent, high quality, crystalline or polycrystalline thin films of Group IB-IIIA-VIA compounds. In this regard, various combinations of thin films of CuSe, Cu2Se, InSe, and In2Se3 are increasingly being studied for use as precursors to achieve the production of large-grained CuInSe2 films for solar cells. An emerging goal is to develop less costly, reproducible processes for making consistent, large-area, high quality CIS and/or CIGS absorber layers that do not involve vacuum deposition technologies. In this quest, thin films of CuSe, Cu2Se, InSe, and In2Se3 that are not made with vacuum deposition technologies would be employed as precursors for the production of CIS and CIGS films for solar cells. These films are also being examined for use in thin film tandem cells and as channel layers in high performance transistors.
The foregoing examples and descriptions of the related art are intended to be generally illustrative and not exclusive. Other examples and limitations of the related art are known to persons skilled in the art or will become apparent to persons skilled in the art upon a reading of the specification and a study of the drawings.